The present invention relates to an optical pickup device, an optical disc device, and a diffraction grating.
As a background art of this technical field is described in JP-A-9-81942. The document includes the description of the problem that “a tracking error signal detection device is provided that has a simple optical system and that suppresses an offset, generated by the translation of the objective lens and the tilt of an information recording medium, and at the same time keeps the maximized amplitude of the tracking error signal even if the track spacing of an information recording medium varies.” The document also includes the description of the solution that “a light-condensing spot, formed by two light beams including one light beam half of which has a phase difference of 180 degrees, is formed on the information recording medium, the light beam from this information recording medium is received by a pair of two-part light detectors, and the tracking error signal of is obtained from the difference outputs”.